P2 leftovers: every inline dialog now audited + one shared sample clamp (review complete bar peer extraction)

Inline-built dialogs were invisible to the accessibility audit - which is exactly how
mnemonic-less buttons kept slipping through (the single-instance dialog Andre caught,
and now TWO more found by this very change: ManualPeerPrompt's OK/Cancel and
ProfileSaveAsPrompt's Cancel had no mnemonics - both fixed). Each inline dialog's
construction is split from its ShowDialog into a Build seam, and the audit now covers
15 dialog surfaces (was 8): manual peer prompt, quick profile switch, change password,
password manager, profile name prompt, and the service Additional-options window join
the eight Form dialogs. Behaviour unchanged.

The encoder-boundary clamp is now ONE shared rule (Core SampleClamp) instead of three
private copies in MixingEngine / AsioCaptureBackend / PushModeWasapiBackend - and the
ASIO copy's up-to-four interlocked increments per frame became one batched add per
buffer on the RT thread. Pinned by a new self-test (over-range clamps to exactly +/-1
and counts; +/-1 exactly passes untouched).

Gate 61/61.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/RemSound.App/ProfilePasswordManagerDialog.cs

@@ -14,10 +14,22 @@ namespace RemSound.App;
 internal static class ProfilePasswordManagerDialog
 {
     public static bool Show(IWin32Window owner, ProfileStore store)
+    {
+        var (dialog, rows) = Build(store);
+        using (dialog)
+        {
+            if (dialog.ShowDialog(owner) != DialogResult.OK) return false;
+            return SaveChanges(store, rows);
+        }
+    }
+
+    /// <summary>Construction split from ShowDialog so the accessibility audit can inspect the real
+    /// dialog (inline-built dialogs used to be invisible to the audit).</summary>
+    internal static (Form Dialog, List<(string Title, string Original, TextBox Box)> Rows) Build(ProfileStore store)
     {
         var titles = store.ListProfileTitles();
 
-        using var dialog = new Form
+        var dialog = new Form
         {
             Text = "Profile passwords",
             StartPosition = FormStartPosition.CenterParent,
@@ -77,9 +89,11 @@ internal static class ProfilePasswordManagerDialog
         dialog.Controls.Add(intro);
         dialog.AcceptButton = okButton;
         dialog.CancelButton = cancelButton;
+        return (dialog, rows);
+    }
 
-        if (dialog.ShowDialog(owner) != DialogResult.OK) return false;
-
+    private static bool SaveChanges(ProfileStore store, List<(string Title, string Original, TextBox Box)> rows)
+    {
         var changedAny = false;
         foreach (var (title, original, box) in rows)
         {